Jim Bradford's son, Steven, and his son's partner, Kitty Spencer, were sitting on their balcony when they saw a "huge dark coloured dorsal fin" in the water.

They Googled it, thinking it might be a Risso's dolphin, but agreed that the dorsal fin was much bigger, and it was black, not grey.

When they told Mr Bradford of the sighting the next day, on June 9, he agreed with his wife, Julie, that it sounded more like an Orca - although he thought the chances of that were "very unlikely".

"But seeing the video last night they are positive it was an Orca they had seen," said Mr Bradford, aged 61. "They didn't get any pictures or video, because by the time they got their phone out it was gone.

"I knew the description of one from watching nature programmes with David Attenborough!"

Another sighting of an Orca has also been reported off the Devon coast yesterday by a boat trips boss.

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Eileen, Neil’s mother, submitted the footage to the NMA where it was picked up by curator James Wright.

James said: “Upon first viewing the footage my mind did jump to it possibly being an orca and what an amazing encounter it would have been to have had. These are amazing marine predators which specialise in many hunting techniques. In theory, they can be found in our waters but one venturing into Plymouth Sound would be quite a surprise.

“Orcas can be found around the British Isles but not in any great numbers, they are normally confined to more northerly areas off the shores of Scotland and the islands of Orkney and Shetland. There has been one record off Plymouth some four years ago, but that was an isolated appearance.

“We did some research and looked at other possible specimens - bottle-nosed dolphins do come into the Sound and one hung around for quite a while in 2015, but these have very different shaped dorsal fins.

"Common Dolphins can be sighted further offshore, especially by dive boats heading out to the Eddystone Reef, but these are much smaller animals and usually in groups.

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"One which could fit is a Rissos dolphin but at the description of the size it would be a large older individual. As Risso’s dolphins grow and age, they lighten in colour and develop scarring, which this animal did not seem to have.

“We circulated the footage with some knowledgeable experienced cetacean experts who all leaned towards this being an orca.

“We cannot be 100 per cent certain without more definitive footage, especially of the characteristic white markings on the black body, but even if this is a sighting of a Risso’s dolphin in Plymouth Sound it is very special.”